Cable television systems have made possible the transmission of many channels of television programs to the homes of subscribers. Instead of being limited to the number of VHF and UHF channels that can be transmitted and received in a given area, the number of channels in cable systems is limited only by the transmission characteristics of the cable itself and the ability to compress the information in television programs into narrower-bandwidth channels. The advent of fiber-optic transmission systems for use in cable television networks has also vastly increased the number of channels available.
Such increase in channels has also given rise to proposals for interactive television systems wherein a subscriber can transmit information or requests back into the system, which information or requests may subsequently affect programs or information directed to such subscriber. There are a wide variety of applications for interactive television systems, such as video games, video catalog shopping, teaching systems, movies on demand and audio programs. Each application can be tailored for an individual subscriber, for example, a subscriber may be able to select the language of the soundtrack in a movie. However, such systems typically require the ability (i) to control specific programs or information sent to each subscriber and (ii) to receive input messages or requests from the subscriber.
A headend-to-subscriber channel does not need the full bandwidth of the usual television cable channel if compression can be used. For example, if a television signal is digitized and compressed in accordance with the MPEG standard, it is possible to transmit sixteen television programs digitally over a single conventional 6 Mhz cable channel. Other program information, such as high-fidelity audio, still video pictures or text can also be sent in compressed form. By using a large number of conventional channels for ITV purposes, such as is now possible with optical fiber distribution systems, together with compression, it is conceivable that hundreds of virtual channels could be made available.
Systems are also becoming available for transmitting compressed and encoded television signals over telephone circuits. In one such system, 1.5 megabits/second of data, sufficient for one compressed and encoded television channel, can be transmitted from a telephone central office over a twisted-pair loop to a subscriber's premises, and 16 kilobits of data can be transmitted back to the central office, together with regular telephone service. Such a loop is known as an asymmetric digital subscriber loop.
With the wide deployment of digital communication systems, standardized digital transmission systems have become available in which data is transmitted in packet form over networks. One such network is an asynchronous transmission mode (ATM) network in which packets of varying length can be sent. Such packet networks can be used for carrying digitized television signals.
Clearly the means now exist by which tailored television signals can be delivered to individual users, such as by cable television systems, telephone networks, packet networks and satellites. Also, a large number of sources are available from which programs and interactive television applications can be provided. However, problems arise in the efficient generation of television signals composed of signals from multiple video and/or audio sources for use in providing interactive television services.